1. Basic Concepts in Reliability Design - nl3prc

[2] Designing for ReliabilityThe wire bonding process, and the failure mechanisms which can occur during the process, aredescribed below.(1) Au-Al Intermetallic FormationWhen bonding Au wire to Al, whereby the bonding surface is subjected to high temperature, aformation of purple alloy (AuAl 2 ), referred to as the “purple plague,” is often observed. In contrast,when the proportion of Au is high, the Au 2 Al alloy formed is referred to as the “white plague.” Thelatter has higher electrical resistance and is mechanically weaker.Since Au and Al have different diffusion constants, voids that degrade the bonding strength areformed at the intermetallic region, and resistance is increased. Bonding degradation will lead toopen bond wires when mechanical or thermal stress is applied. However, such degradation can beconsidered negligible if the heating process is properly controlled during the manufacturingprocess.(2) Mechanical StressWith plastic-encapsulated devices, temperature cycling can cause excessive mechanical stress onbond wires due to the difference between the thermal expansion coefficients of the plastic resinand the bond wire. In some instances, this can result in an open bond wire. Also, wire deformationduring molding can cause a situation in which a relatively high temperature will cause wires tocome into contact with each other or with the edge of the chip, resulting in an electrical short. Toprevent such problems, specific measures are taken involving optimization and/or automation ofthe assembly process, such as exerting control over the wire loop shape.In hermetically sealed devices, bonding wires can open due to shock or vibration. This should betaken into account, particularly when ultrasonic cleaning is to be performed.Furthermore, mechanical damage during bonding can, in some instances, cause secondary devicefailure. However, this problem can also be counteracted by optimization and/or automation of themanufacturing process.2-31